Given their versatile properties, synthetic polymeric materials have been employed in the biomedical field. Specifically, because of the biocompatibility and biodegradability, aliphatic poly(ester)s, such as poly(lactide) (PLA), poly(glycolide) (PGA), poly(ε-caprolactone) (PCL) and their copolymers, have become increasingly attractive in the design of temporary synthetic scaffolds in tissue engineering.
Their properties and degradation profiles can be precisely tuned to match the needs of the final application. Although their physical properties can be modulated via copolymerization, a major limitation to their application in highly specialized areas, such as the biomedical field, is the absence of readily accessible side-chain functionalities. The bio-functionalization of polyester-based scaffolds with biologically relevant ligands could provide a host of opportunities to control cell adhesion and functions. Specifically, conjugation with a peptide containing the sequence arginine-glycine-aspartic acid (Arg-Gly-Asp, or RGD) has been shown to improve the cytocompatibility and cellular attachment characteristics of temporary polymeric devices by promoting cellular adhesion through binding to integrin receptors. Therefore, the development of simple and controlled chemical synthetic approaches that allow the preparation of functionalized poly(ester)s is one of the main topics in this field.1 
Two strategies can be followed to obtain polyesters with functionalities incorporated as side groups. First, post-polymerization modifications have been used to modify the surface of the polymers without impacting the bulk; however, these modifications are sometimes associated with side reactions, such as chain scission, with a consequent deterioration of the polymeric features.
The second method, co-polymerization with functionalized monomers, allows the preparation of editable polymers through the polymeric chain, which can affect the material in the bulk. Following the Kimura's pioneering approach, functionalized lactide- and glycolide-type monomers featuring pendant-protected carboxyl, hydroxyl and amino groups have been prepared by diazotization of available amino acids, such as aspartic and glutamic acids, serine or lysine, into the corresponding α-hydroxy acids, followed by cyclization.2,3 Cyclic di-esters carrying aliphatic groups have also been obtained from their corresponding α-hydroxy acids. Attempts to obtain the analogous hydroxy acid starting from the diazotization reaction of the cysteine were unsuccessful.
Thiol synthesis, modification and functionalization are highly attractive and efficient in polymer and materials science and have immense application in biological therapeutics and drug delivery. The abundance of the thiol-based amino acid cysteine may allow the use of thiol chemistry to easily conjugate polymers with peptides or proteins.4 Moreover, the thiol-ene click reaction represents an efficient tool for further polymer modifications. Following the example of nature, where disulfide bond formation plays an important role in the folding and stability of biopolymers, the oxidation of thiols into the corresponding disulphides should also be exploited as stimuli-responsive linkages to obtain improved and intelligent materials. Different approaches for the preparation of poly(ester)s with mercapto groups have already been reported. Exploiting their chemical structure, PCL samples functionalized with a thiol group on the chain-ends have been prepared.5 Additionally, amphiphilic PLA-based block copolymers functionalized with disulfides at the block junctions have been described.6 Alternatively, poly(ester)s with thiol pendant groups grafted throughout the polymeric chains have been obtained by polycondensation reaction approaches, enzyme-catalyzed chemoselective reactions of mercaptosuccinate with different diols,7 or the polycondensation of dicarboxylic acid-containing thiol groups and diols in the presence of a metal initiator.8 In this regard, we previously reported the polycondensation of suitably prepared sulfur-functionalized hydroxy acids, which afforded low molecular weight samples.9 
In view of orthogonality conflicts and considering their instability toward oxidation and incompatibility with many polymerization processes, several strategies to protect thiols and thus prevent unwanted reactions have been developed and optimized.10 We have also previously described a route to aliphatic poly(ester)s with thiol pendant groups.11 